Numerical expressions are exactly what i was looking for, amongst others a way to average ratings so i don't have to do this manually. I'm not bothered too much with speed as these can be calculated and cached in another field for faster display in view schemes.
When thinking about how to do this, how about "group by", since expressions can only carry out operations on one item at at time and are not able to take into account values from other items.
So group by [Field] could give a way to select a [Field] that was common between items and then carry out a calculation.
This would work with
associative math, like addition or multiplication where the order that the items are presented in, does not affect the result.
(5+2)+1 = 5+(2+1) or 5 x ( 2 x 3 ) = (5 x 2 ) x 3
It gets trickier with non-asociative math like subtraction & division where the order of items matters.
(5 - 2) - 3 does not equal (5 - 3) - 2
maybe that could be solved using parenthesis, to indicate order of calculation.